Process for producing styrene



Jan 1,1946- C. R. MccULLoUGH ET A1.' 2,392,289

l PROCESS FOR PRODUCING STYRENE 5 Sheets-Sheet 1 5 Sheets-Sheet 2 n w uw# W @Ibi/o p a 00V C. R. MCCULLOUGH ET AL PROCESS FOR'PRODUCING STYRENE Filed Sept. 8 1945 C'/V/ICT 77/1//5-"0 0.,? 5ft Jan. l, 1946.

' Jan. 1, 1946. .Q R MccULLoUGH ET AL 2,392,289

PROCESS FOR PRODUCl-.NG STYRENE I Filed'sept. 8, 1945 s sheets-sheet 5 CoA/m67 WMP-0.3 5M..

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i 20 407g "L: D: X K u) /l u JU N @an 64 0' 660 700 74 "o ma l v lNvENToRS F19 5 l CAM/05m A24/66011006# ATTO N EY Piramedia@ 1, 194s UNITED STATES vPATENT OFFICE y 2,392,289 PROCESS FOR PRODUCIN G STYBENE campbell n. Meouuogn ana wmara n. Geni-ke, Dayton, Ohio, assignors to Monsanto Chemical Company, a corporation of Delaware Application vSeptember 8, 1943, Serial No. 501,552 s claims. cl. 26o-669) the highest yield obtained by the strictly thermal del'wdrogenation of ethylbenzene is about 55% and is obtained when the conversion is quite low, i. e., up to 2,5%. It has beenfound that'conversion is a function ,of time and temperature. 'I'hus for a. temperature of 600 C. and a contact time of 3 seconds, 10% conversion is obtained, whereas at a temperature of 740 C. a conversion of is also obtained in 0.03 second.v Within these limits the yield is dependent entirely upon conversion, as far as can be determined.

In prior attempts to increase the yield and conversion of styrene, various addition materials (hydrogen acceptors) have been added tothe It is recommended in this patent that approximately 2% of sulfur based on the total mixture, or 4% of sulfur based upon the ethylbenzene in the mixture undergoing pyrolysis be employed.

In U. S. Patent 1,997,967 to Gibbons and Smith additional studies on the effect of various addition agents to the pyrolysis mixture are reported. As muchras 6 parts or sulfur per 100 parts of ethylbe m were employed by Gibbons and sions are reported when smaller quantities of sull-ur were used. For example, when three parts of per 100 parts of ethylbenzene were employedithe conversion was 31.6% and the yield was 41,8%, with two parts of sulfur the convert.

sion was 27.9%and the yield was 43.1%,-while reported when this amount or sulfur" .10 tionship between yield and conversion shows that 'with one part of sulfur the conversion `and yield were respectively 24.2% and 36.0%. As a whole, however, the work reported by Gibbons and Smith using sulfur or, for that matter, any other addition agent does not show any better yields and conversions than maybe obtained using strictly thermal methods.

The experimental work of Gibbons and Smith as well as Smith, mentioned above, was carriedout by heating ethyl-benzene vapor in an electrically heated -iron or Chromel tube 3 Ieetlong and it inch in internal diameter. The rate of iiow of ethylbenzene into the furnace was apl above tube.

proximately grams per -hour through the time of approximately 16 seconds. 4

It is obvious from the above discussion of the prior art that such prior art processes were limited to the addition of sulfur in amount up to 6%, .based upon th'ej ethylbenzene', while contact times were relatively long.

We have now/ found that if the dehydrogena- `tion of ethylbenzene 1:0l styrene be carried out in the presence of much larger amounts of sulfur than was employed in the prior art, and under conditionsv such that the time of exposure, ats reaction temperatures, of the ethylbenzene to the sulfur vapor. be relatively short, that muchhigher yields and conversions of styrene may be obtained. We have also determined that the larger quantities is realized if the sulfur be rst vaporized and the vapors thereof :be then mixed with the hot ethylbenzene vapors. Thus in the preferred mode of operation, according to our invention, the sulfur and ethylbenzene are separately vaporized, the ethylbenzene vapors being heated to temperatures suiiiciently high to cause incipient dehydrogenation, and the sulfur and ethylbenzene vapors then mixed together. The mixture of vapors so formed is Vmaintained at This rate of dow will give a contact v greatest effectiveness of sulfur employed in these reaction temperatures for- 0.03 second to 2.5

seconds, at the end of which period the reaction is quenched by's'harply cooling the gases. A liquid condensate containing the styrene is obtained from the cooled gases.

The sulfur vapor, prior to mixing Ivvitlr'the ethylbenzene should be heated to a. temperature of atleast 500 C. and may even be heated to temperatures as high as the reaction temperatures which temperatures may bebetween 600 C. and 780 C.- The proportion of sulfur employed with respect to the ethylbenzene has been found to be critical and high yields of styrene may be obtained when sulfur is employed in amoxmts ranging from lbs. to 25 lbs. per 100 lbs. of ethylbenzene, i. e., 10% to 25% based upon the ethylbenzene with which it is to be mixed for the reaction.

, The ethylbenzene vapor should be heated prior to mixingvwith the sulfur vapor, to temperatures within the range of from 600 C. to '180 C. The vapors of sulfur and ethylbenzene are then combined and the resulting mixture of vapors allowed to interact at the reaction temperature for a period of time prior to cooling, the length of such period beingfadvantageously proportioned to the reaction temperature. Thus higher tem- Contact time, seconds Temlmre' 0.03 t0 n l B30-780 0.1 t0 n 5 620-740 0.5 t0 2 5 (5m-660 Our .preferred process may be carried out asv indicated on the diagrammatic flow sheet constituting Figure 1 of the drawings. In Figure 1, numeral I represents a source of liquid ethylbenzene. Pipe 2 provides a means for conveying the ethylbenzene from the storage vessel to a flow measuring device or feeder 3, wherein the ow of *liquid ethylbenzene is measured and controlled. The measured ethylbenzenethen passes by pipe 4 to vaporizer 5 in which, by the application of heat, the liquid ethylbenzene Iis converted to Vapor at a temperature'of about 150 C., which temperature represents afsuperheat of about C. above the normal boiling point of this material. The vapor then ows by pipe 6 to preheater 1, Where additional heat is added and the vapor attains a temperature of, say,

500 C. Atthis point the preheated vapors are conveyed by pipe 8 to a superheater 9 where the temperature is further raised to the temperature passes by means of pipe I8 into vaporizer- I9.

. At this point the molten sulfur is vaporized and a slight degree of 'superheat imparted thereto, after which the sulfur vapors 'pass by means oi pipe 20 into a further4 superheater 2|. In some cases, which are explained more fully below, it may not be necessary to eniploy superheater 2l and for this reason it is indicated in the flow sheet by dotted'lines. If, however, the superheater 2I is employed the temperature of the sulfur is raised thereby to a temperature approximating the temperature of the converter and passed into the converter II by means of pipe -In case superheater 2| is not employed, the temperature of the sulfur vapor is raised by the vaporizer to at least in excess of 500 C.

Converter Il is vmerely an empty chamber which may be constructed of stainless steel and provided with a brick or other refractory lining in order to protect the steel from the action of the heated sulfur vapors fwithin the chamber. v

cooled by contact Vwith c old surfaces which are in turn cooled by means of a cooling fluid such as water. While it is not necessary that the coolingbe sulliciently eilective to cool vthe gases, in-

the rst instance to temperatures low enough to cause condensation of the condensable vapors, it

V should be sufficientlydrastic so that further reat which pyrolysis begins. Because of the thermal instability of ethylbenzene at temperatures Y above 60.0 C, a superheater is employed wherein a short contact time may be obtained which permits the ethylbenzene to be lheated well within the temperature range, at which it will normally undergo dehydrogenation. However, because of the short contact time employed the heating of the ethylbenzene maybe eifected without sub'- stantial dehydrogenation in the superheater. preferred temperature ior the operation of our process is within the range of' from 650 C. to' 680 C., although the overall workable range of temperature for our process is within the limits of 600 C. to '780 C.

The superheater .ethylbenzene vapors produced in superheater 9 passby means of -pipe I0 4into converter I I, where the reaction with sulfur takes place. A

The sulfur supplied to ourprocess originates at.

a source indicated in the iiow sheet by numeral I2. It passes thence to sulfur melter I3 and the' molten sulfur then passes by pipe I4 to sul.

fur feeder I5. The sulfur feeder measures and v controls-the flow of molten sulfur passing by means of pipe I6 into preheater I'I.I Here the temperature of the molten sulfuris raised to about 440 C. or to a temperature just short of the boiling point, after which the liquid sulfur actions in the hot gases are quenched. Such quenching of the reaction represents the termination of the contact period. The initiation of the reaction is, of course, caused by the mixing together of the hot vapors in the converter. The condenser 24 produces a liquid condensate which leaves the condenser by pipe 25,'which condensate is then treated byconventional means, not shown, for recovery of contained styrene. The gaseous products consisting mainly of hydrogen sulfide and hydrogen pass out of the system by pipe 26 to recovery or disposal means, notI shown in the drawings.

Considerable latitude is possible in the operation of our process. Because of the separate feeding means provided, an adjustment and control of the desired amounts ofjethylbenzene and sulfur processed in the converter may be made and we have accordingly found4 that the best results are obtained when the amount of sulfur employed is maintained within the range of from 10 lbs. to 25 lbs. of sulfur per 100 lbs. of ethylbenzene owing to the converter. Within this range of sulfur content, the reaction produces styrene at a yield of from to 87% and even ashigh as %,-With a conversion of from 25% e Without limiting ourselves to a precise theory,l

to some extent other organic sulfur compounds y such as carbon disuliide vand traces of mercap-v tens. As a result ofthe combination of sulfur with the hydrogen the equilibrium is displaced in the-.direction of greater concentrations of styrene in the reacted gases, without forming increased amounts of by-products.

The beneficial results of separately heating the ethylbenzene and sulfur are two-fold. One advantage is to permit higher preheating temperatures for the ethylbenzene to be attained outside of the converter than is p ossible when the ethylbenzene and sulfur are first mixed and then heated together. A second .result is to substantially decrease the formation of by-product tarry bodieswhich represent a material loss to the process.

Because of the rst advantage mentioned above, vit is possible by means of the higher prehea'twhich may thus be obtained to operate the converter without supplying additional heat to the converter itself. In other words the operation of the converter approaches a substantiallyA adiabatic state. When this condition is reached (or approached) the operation of the converter proceeds solely by means of the heat present in the reacting gases supplied thereto. Since the overall reaction heat change of the reaction taking place in the converter is to some extent endothermic, it is desirable that the gases be introduced into the converter at a sufficiently elevated temperature so that the final tempera- Ature of the gases leaving the converter is still well within the reaction range, i. e., above 600 C. Thus the possibility of supplying a large proportion of the necessary heat to the converter by means of superheated ethylbenzene 4vapor may, under some conditions of operation, make unnecessary' the supplying of further additional preheat tothe sulfur above about 500 C. by means of sulfur preheater 2l in Figure 1 of the drawings. The supply of some additional heat from an external source to converter Il is, of course, not precluded.

By separately vaporizing and preheating theA etlwlbenzene and the sulfur prior to mixing in the converter a substantial decrease of the byproducts tars is obtainedl over that which is obtained when vaporizing and preheating a mixture thereof. This effect is illustrated in the followdEnSatE 2 Vaporizing and reheatingt esulfur and ethylbenzene sepately before mixing Vaporzing 4and preheating the mixture of sulfur and ethylbenzene Percentage of styrene in condensate The heat required by the converter Il of Figure 1 after it has reached a steady state, and neglecting heat losses to its surroundings, is, of course, the algebraic sum of the several heats of reaction for the reactions taking place therein. The principal reaction is, ofcourse, the dehydrogenation of ethylbenzene vforming styrene ,and

hydrogen. This reaction is an endothermic one. The second important reaction is the formation of hydrogen sulde by reaction of the liberated hydrogen and the sulfur present. This reaction is exothermic, although it is not sufficiently so to supply all of the .heat absorbed or required in the principal endothermic reaction. In addition to the main reactions there are various minor secondary reactions which take place with respect to which the thermal characteristics are unknown. On the whole, however, it may be said that the net overall thermal effect of the reactions in the vconverter is a slightly endothermic one. This means that the temperature of the gases leaving the converter will generally be lower than the temperature of the gases entering the converter.

As pointed out above, by carefully controlling the various factors of our process, it is possible to obtain yields of styrene from ethylbenzene better than 85% and even somewhat over 90%.

In Figures 2 and 3 the abscissae give a temperture of the ethylbenzene vapor entering the converter, while the ordinates give the pounds charged. The lines in the diagram marked a denne the approximate fields wherein the yield of styrene obtained is within the stated yield -ethylbenzene entering the converter should be carried to a temperature between about 664 C. and 710 C. The sulfur vapor is heated to a temperature wtthin the range of 500 C. to 710 C. The amount of sulfur mixed with the ethylbenzene should be between about 16 pounds and 20.5 pounds (16% and 20.5% based upon the ethylbenzene), preferably about 18 pounds of sulfur'` per pounds of ethylbenzene.

Figure 3 illustrates the performance'of the converter utilizing the somewhat longer contact stime of A0.3 second. Ethylbenzene vapor should be heated to a temperature of from about 648 C. to 684 C. and then mixed with heated sulfur vapor in amount varying between about 11 pounds to about 22 pounds (11% to 22% of sulfur based upon the ethylbenzene), preferably about 16.5 pounds of sulfur per 100 pounds of ethylbenzene.

What we claim is:

1. The process which comprises separately n heating a stream of sulfur `vapor and a stream of ethylbenzene vapor, saidethylbenzene being heated to a. temperature at which a. dehydrogenation reaction occurs, mixing said sulfur and ethylbenzene together in the proportions of between 10% and 25% sulfur based on said ethylbenzene maintaining the resulting mixture at a temperature of from 600 C. to '180 C. for a period of from 0.03 vsecond to 2.5 seconds and then cooling said mixture. v A

2. The process which comprises heating a. stream of ethylbenzene vapor to a temperature between 600 C. and 780 C. and then mixing said ethylbenzene vapor with sulfur vapor in the proy pontion of more than 10% but less than 25% by weight` of sulfur based on. said ethylbenzene,

maintaining the resulting mixture at a'tempera ture of between 600 C. and 780 C. for a period of at least 0.03 second, cooling said mixturetof ethylbenzene and sulfur after an interval of less than 2.5 seconds contact therebetween, and recovering` a condensate from said mixed vapors.

3. The process deiined'in claim 2, in which the temperature of said mixture of ethylbenzeneand sulfur is maintained within the limits of 650 C.l

to 680 C. l

4. The process for producing styrene which comprises heating a stream of ethylbenzene vapor to a temperature between 650 C. and 680 C..

separately heating a stream of sulfur vapor to .a

5. The pr'ocessfor producing styrene which comprises heating a stream of ethylbenzene vapor to a temperature between about 664 C. Aand 710 C., separately heating l a stream of sulfur vapor t0 a temperature above 500 C., mixing said assenso sulfur vapor with said ethylbenzene vapor in the proportions of between 16 pounds and 20.5 pounds of sulfur per 100 pounds oi' ethylbenzene,l and after aperiod oi' about 0.05 second cooling said vapors and thereafter recovering styrene from said cooled vapors. I

6. In the process for producing styrene by the dehydrogenation of ethylbenzene, the step which comprises heating together at a temperature between 600 C. and 780 C., andmfor an interval Aoi' more than 0.0.3 second, but less than 2.5 seconds a mixture of ethylbenzene vapor and sulfur vapor,

the amount of sulfur comprising between 10% and 25% by weight of the ethylbenzene in said mixture. y

7. The process defined in claim 6. in which the mixture of vapors is maintained at a temperature between 630 C. and 180"v C. for a period of between 0.03 and 0.1 second.

8. The process which comprises heating a stream of ethylbenzene vapor to a temperature between 600 C. and 780 C., separately heating a stream of sulfur vapor to a temperature in excess of 500 C., but below a temperature such that when mixed together with said ethylbenzene the temperature of said mixture will be below 780 C., mixing together said ethylbenzene vapor and-sulfur vapor streams in the proportion of between 10% and 25% by weight of sulfur based on said ethylbenzene andafter an interval of more than 0.03 second but less than 2.5 seconds,

during which interval the mixture is maintained in a -thermally insulatedreaction zone.."coollng said mixture and recovering a condensate from said vapor.

CAMPBELL R. MCCULLOUGH. WILLARD H. GEI-IRKE. 

